Translocation

Form of chromosome mutation in which parts of chromosomes are exchanged both within a chromosome and between chromosomes. As with other mutations, this changes the structure of the chromosome. This can result in an altered sequence of genes - with specific consequences.

• In a balanced translocation, chromosomes or a part of them are transferred to another chromosome. This translocation is balanced if the total amount of genetic material is unchanged and no gene is damaged at the chromosome breakpoints. This mutation does not affect the phenotypic characteristics of the organism.
• In the case of an unbalanced translocation, the amount of genetic material is changed by the loss or addition of a chromosome or part of a chromosome. An example of this is translocation trisomy 21, where the inheritance of translocation chromosomes can result in a genome with excess or missing chromosome parts (partial trisomies or monosomies). This mutation can cause numerous phenotypic anomalies and malformations.
• Reciprocal translocation involves a reciprocal exchange of non-homologous chromosomes, whereby the total number of genes remains the same. The translocation is balanced (e.g. Philadelphia chromosome).
• In centric fusion or "Robertson translocation", two so-called acrocentric chromosomes combine to form a new, larger chromosome. This mutation can be balanced or unbalanced. In this translocation, a centromere and the short arms of the chromosomes involved (these are without genetic information) are lost (see illustration). Acrocentric chromosomes are chromosomes 13, 14, 15, 21, 22. These chromosomes can fuse centrally with each other.

• In the so-called Philadelphia chromosome, a reciprocal translocation occurs (t(9:22)), caused by a translocation of the main part of the long arm from chromosome 9 to chromosome 22. The Philadelphia chromosome is detected in > 95 % of patients with chronic myeloid leukemia and in 5 % of all children and 20 to 30 % of adults with acute lymphoblastic leukemia.
• In Burkitt's lymphoma, a reciprocal translocation between chromosomes 8 and 14 is observed (t(8:14)).
• In mantle cell lymphoma, a reciprocal translocation occurs between chromosomes 11 and 14 (t(11:14)).

In genetics, a short form is used to define a translocation exactly. For example, a translocation (t) 46,XY,t(2;6)(q21;p13) would be interpreted as follows: Band 1 in region 2 of the long arm of chromosome 2 has been exchanged with band 3 on region 1 of chromosome 6.